"Yeah, I know that song, it's in the key of A, right?"
"Nah man, we do that one in D."
You need to know how to transpose music. Everyone does. When you play music with other people there's no guarantee that they will play it in the same key that you're used to. This could be because:
There are essentially two ways to do this: by the numbers, and by the intervals. I use both on a regular basis. In fact, transposing is such a common thing, that sometimes I will transpose music on the fly and read in one key while playing in another! (That is if it's slow and simple!)
I'll explain both ways because one way or the other might be easier depending on the situation.
Here is a phrase we'll use for this example.
Let's start with the numbers method.
Basically, this way involves assigning each note a number, and figuring out what that number means in the new key. This is similar to the way the Nashville Number System works, but we can also use it with notes instead of just chords.
Let's move this phrase from the key of D to the key of A.
What we're going to do first is assign each note in the current scale (the scale we're moving from) a number or "scale degree".
We then label each note according to its scale degree. All of the Ds are 1s, all of the Es are 2s all of the F#s are 3s, etc.
Now it's time to move to the new key. We basically just do the last process in reverse. We're going to take the numbers and turn them back into notes. First we need to figure out what the numbers mean in the new key. To do this, we again will assign a number or "scale degree" to each note of the new scale.
Now we use this new scale to apply these notes to the numbers, and we get this.
Ta-da!!! That's it! This method will allow us to transpose music from any key to any other key.
If we were to try the key of E:
We would get this:
Now let's look at the other way.
Here we don't have to go through the middle step of converting to numbers. Instead we are basically going to find the distance between the two keys, and add (or subtract, depending on which direction we're going) that distance to every note.
Let's start with the easy key change and move our phrase from D to E.
First we need to find the interval between the two keys (or the distance between them.)
If we count in half-steps or whole-steps, we can measure the distance from the root note of one key (D) to the root note of the other key (E). In this case, it's one whole step.
This will be the interval that we will add to each note.
Now we go through the music and add one whole step to each note, so we go from this:
Now let's try moving it from D to A. The distance from the root note D to the root note A (again, counting whole- and half-steps) is 3.5 whole-steps or 7 half-steps. We now add this interval to each of the notes (Ds become As, Es become Bs, F#s become C#s, etc.) and we go from this:
This method can get kind of tricky sometimes, like when moving from sharp keys to flat keys for example, but it still works the same way.
Deciding which way to transpose music depends a lot on personal preference. Some people work better with intervals and don't want to go through the extra step of the numbers. Some people like the universality of the number system.
I use both. Notice how in the interval example we started with the D to E change? The smaller interval is easy because we just play one whole-step higher. With most instruments this is not a difficult change. Moving from D to A is a little harder using the interval method because the note we're moving to isn't "right next door". In this case I would probably use the numbers method.
Both of these methods to transpose music take some practice, but after you've been working at it for a while it becomes much easier.
Hope this helps! Practice hard and let me know if you have any questions!